In conventional video display devices that allow an observer to observe an image (a virtual image) by illuminating a display element by means of an illumination optical system and guiding image light from the display element via an eyepiece optical system to the observer's eye, an axisymmetric (rotational-symmetric) lens or a non-axisymmetric (non-rotation-symmetric) cylindrical mirror is adopted in the illumination optical system. An example of the configuration with an axisymmetric lens is disclosed in Patent Literature 1, for example, and an example of the configuration with a cylindrical mirror is disclosed in Patent Literature 2, for example.
As for a case where a linearly arranged plurality of light emission points (such as LEDs that emit red (R) light, green (G) light, and blue (B) light) are used as a light source, there is a technique for reducing unevenness in color by disposing a diffuser plate (a unidirectional diffuser plate) which diffuses light in the direction in which the light emitting points are aligned. In such a case, if an axisymmetric lens is used in the illumination optical system, a desirable configuration for more efficient use of light is such that an optical pupil formed by the eyepiece optical system and the light source are substantially conjugate with each other with respect to a direction in which the illumination light is not diffused by the diffuser plate. Thus, the curvature of the lens with respect to that direction is first determined, and then, based on this curvature, the curvature of the lens with respect to the direction in which the illumination light is diffused is dependently determined (since the lens is axisymmetric, this curvature is equal to the curvature determined with respect to the direction in which the illumination light is not diffused). On the other hand, in the case where a cylindrical mirror is used in the illumination optical system, the optical pupil and the light source are allowed to be substantially conjugate with each other by disposing the mirror such that it has a curvature in the direction in which the illumination light is not diffused by the diffuser plate and such that it does not have a curvature in the direction in which the illumination light is diffused by the diffuser plate.